[PATCH 0/2, RFC] xfsprogs: try to handle mkfs of a file on 4k sector device

[Eric Sandeen <sandeen@xxxxxxxxxx>]

Today if you mkfs.xfs <filename> where the file resides on a
hard-4k filesystem, we fail because it tries to do 512 direct
IO when 4k is required; this is a bit cryptic:

# mkfs.xfs -f mnt/fsfile
meta-data=mnt/fsfile             isize=256    agcount=4, agsize=8192 blks
         =                       sectsz=512   attr=2, projid32bit=1
         =                       crc=0
data     =                       bsize=4096   blocks=32768, imaxpct=25
         =                       sunit=0      swidth=0 blks
naming   =version 2              bsize=4096   ascii-ci=0 ftype=0
log      =internal log           bsize=4096   blocks=853, version=2
         =                       sectsz=512   sunit=0 blks, lazy-count=1
realtime =none                   extsz=4096   blocks=0, rtextents=0
existing superblock read failed: Invalid argument
mkfs.xfs: pwrite64 failed: Invalid argument

We can modify platform_findsizes() to use the fsgeom call to get the
"sector size" which should be used here, and warn that mismatches
might exist if it fails.

This does mean there'll be a new warning emitted on fs images hosted
on non-xfs filesystems; I'm not really quite sure it's worth it,
hence the RFC nature of this lightly tested 2-patch series...